Several types of sensors including resistive, capacitive and magnetic are used to measure force and moments. The earliest resistance sensors were thin strands of wire having ends attached to an object subject to strain. As the wire was stretched, measured change of electrical resistance of the wire indicated strain in the object. The "capacitive" sensor comprises a diaphragm being one electrode of a capacitor whose capacitance varies in proportion to pressure applied to the diaphragm. Semiconductor technology has been adapted to provide far more sensitive sensors based on both resistance variation and capacitance variation. The ability to manufacture these devices using integrated circuit techniques has increased the variety of applications of force sensors on a broad industrial front.
Wilner U.S. Pat. No. 4,793,194 discloses a strain sensitive element including an n-type silicon substrate with grooves defining two islands and a frame wherein the grooves form a relatively flexible hinge connection between the islands and frame. A pair of strain ensors and a pair of conductors bridge across the groove and connect the islands and frame. All parts are integrally formed by an etching and diffusion process. The diffusion process with Boron converts the bridges to p-type silicon rendering them strain sensitive.
Yamagishi et al U.S. Pat. No. 4,841,272 discloses a strain gage comprising an n-doped Si substrate with Ge atoms diffused therein to reduce temperature coefficient.
Guckel et al U.S. Pat. No. 5,188,983 discloses a resonating beam of polysilicon which is piezoresistive. Resonant vibrations are induced by a variety of ways such as by capacitive induction. The beam is formed by depositing the beam on a sacrificial layer that has been formed on a substrate then etching away the artificial layer.
Peterson et al U.S. Pat. No. 5,231,301 discloses a sensor comprising an n type semiconductor substrate having a p type piezoresistor bridge over a flexible region and a metallic guard shield over laying the piezoresistor and electrically connected to the n-doped substrate such as to prevent charge buildup.
Techniques have been disclosed for forming a sensor depending on capacitance variation of a semiconductor diaphragm facing a substrate and enclosing a chamber.
For example, Mastrangelo U.S. Pat. No. 5,369,544 discloses a silicon on-insulator capacitive sensor formed by surface machining in which a flexible silicon diaphragm forms a sealed compartment with a semiconductor substrate.
Foyt et al U.S. Pat. No. 5,375,034 discloses an ion milling technique for depositing a glass spacer to form a capacitive sensor.
Xiang-Zheng et al U.S. Pat. No. 5,242,863 discloses a method for producing a piezoresistive diaphragm enclosing a chamber over a silicon substrate.
For some applications, there is a requirement for an arrangement of more than one sensor such as for measuring and correlating strain in various locations of an object.
For example, U.S. Pat. No. 5,227,760 discloses a strain gage comprising four gages connected as a Wheatstone bridge with resistance means for adjusting the offset voltage of the bridge.
Braouwers et al U.S. Pat. No. 5,144,841 discloses a hollow bolt with a piezoresistive element encapsulated within the the hollow bolt.
Discovery of materials other than semiconductors and having piezo resistive properties have been disclosed.
Alles U.S. Pat. No. 5,297,438 discloses a ferroelectric piezoresistive compound.
Rubner et al U.S. Pat. No. 4,808,336 discloses a piezo resistive blend of a doped acetylene polymer and an elastomer.
Some situations require a gage which is capable of indicating force or moment applied in any one of a number of directions. According to present art, such gages require independent bonding of a plurality of strain sensors, each arranged to respond to any one of the directions of applied force. The sensors are typically arranged as a three dimensional array which are more expensive to construct than the device of the present invention.
Sensors having a silicon based structure can be manufactured only on a micro scale and have minute force bearing capability; hence they can be used only at the scale of other 'silicon based micro-machined components and assemblies. Such sensors are also inherently built as two dimensional planar components, sensitive to forces either normal to or aligned with the structural plane. Therefore, measurements are usually limited to three force components such as F.sub.z, normal to the plane of the sensor and its moments Mx and My or F.sub.x, F.sub.y, and their moment M.sub.z. These forces and moments are generally too small to be humanly perceptible.